Rubber products today are made from natural rubber and synthetic rubber or blends thereof. Natural rubber (NR) differs from synthetic rubber (SR). Natural rubber is made from the milk of the rubber tree. This rubber milk, called latex, is a colloidal dispersion in an aqueous medium. Only small percentage of the latex is used directly and the greatest part is processed into hard rubber. Among many different types of synthetic rubbers, the most common synthetic rubbers are SBR (styrene butadiene rubber), BR (butadiene rubber), EPDM (ethylene propylene diene rubber), IR (isoprene rubber), IIR (isoprene isobutylene rubber), NBR (acrylonitrile butadiene rubber), SIS (styrene isoprene styrene), SBS (styrene butadiene styrene) and CR (poly-2-chlorobutadiene). Depending on the production procedure, one differentiates between polymerisates, polycondensates and polyaddition synthetic rubbers.
Rubber (raw rubber) is a non-crosslinked, but cross-linkable (curable, vulcanisable) polymer with rubber-elastic properties at room temperature. Elastomers are polymeric materials that are crosslinked (vulcanized). They are hard and glassy at low temperature and do not exhibit viscous flow even at high temperature. Vulcanization is a process in which rubber, thorough a change in its chemical structure, is converted to a condition in which the elastic properties are conferred or re-established or improved over a greater range of temperature. Examples of rubber products are unwoven textiles from bonded fabrics impregnated with a latex mixture, form products including rubber-metal-compounds such as shock absorbers, elastomers prings, coupling parts, hangers, glues, repair material, foam, micro cellular and cellular rubber products, conveyor belts, flat and vee-belts, tires, sole material, sheet material, and punching products.
Rubber products are frequently made up of several rubber layers each with the same or a different chemical composition. During product manufacture these layers must adhere to one another adequately in the pre-vulcanized state. For example, an assembled tire blank is required to hold together for a fairly long period prior to vulcanization. It is therefore important that the rubber mixtures used have an adequate “tack.” The property termed “tack” is defined as the force required to pull apart two pre-vulcanized rubber mixtures which have been pressed together under certain defined conditions. While natural rubber mixtures generally have good tackiness, mixtures of synthetic rubbers are much less tacky and, in extreme cases, possess no tackiness at all. Therefore, it has been common practice to add a tackifier to less tacky rubbers or rubber mixtures to increase their tack. In synthetic rubber products, synthetic rubber adhesive compositions are employed to improve tack and provide good cured adhesion. Various compositions have been proposed.
Rubber compositions containing a tackifier are generally formulated in internal mixers or on sets of rollers from a natural or synthetic rubber (e.g. styrene-butadiene copolymers, polybutadiene) or mixtures thereof, fillers, processing agents and vulcanizing agents. After formulation, the rubber composition is then used in manufacturing to produce the desired rubber product. This includes additives such as reinforcing resins for rubber and elastomers, as tackifiers for rubber, as bonders for friction linings, as curable molding compositions, impregnating agents, coatings, paints, and as binders for fine-particle inorganic substrates. The rubber composition must remain sufficiently tacky during the manufacturing process, even when the process is interrupted for fairly long periods, which is not unusual particularly when manufacturing involves processes at different locations requiring the storage and transport of pre-finished goods.
Known tackifiers include, for example, colophony, hydrogenated and dimerised colophony, terpene resins and modified terpene resins, hydrocarbyl resins based on unsaturated C5 hydrocarbyls, unsaturated C9 hydrocarbyls, dicyclopentadiene or coumarone, phenolic resins of the novolak type, for example those obtained from hydrocarbylphenols having from 4 to 15 carbon atoms in the hydrocarbyl groups by reaction with formaldehyde in an acid medium, and hydrocarbylphenol resins prepared by reacting hydrocarbylphenols with alkynes, particularly acetylene.
Novolak resins are well known tackifiers used in a large variety of rubber compositions. Novolak resins have a repeating structure of phenolic units obtained by reacting phenolic compounds with aldehydes in the presence of an acid catalyst. Differing in chemical structure a variety of phenolic novolak resins have been combined with rubber formulations as tackifier resins. Examples of the variety of novolak resins used as tackifiers include a phenol-crotonaldehyde novolak, the phenolic component of which may be, for example, an alkylphenol and/or a diphenol, e.g. resorcinol or hydroquinone (U.S. Pat. No. 4,167,540); a modified novolak resin comprising terpenes and unsaturated carboxylic acids, and/or derivatives of these compounds (EP-A 0362 727); a phenolic resin referred as a xylok resin comprising a phenol compound and an aralkyl such as p-xylylene glycol dimethyl ether (Japanese Patent Publication No. 14280/1977); or a modified phenol-aralkyl resin obtained by reacting phenols, an aralkyl compound and formaldehyde in the presence of an acidic catalyst (U.S. Pat. No. 6,642,345).
As a further example of the variety of novolak resins, U.S. Pat. No. 4,889,891 discloses an alkyl substituted highly branched resorcinol formaldehyde novolak. U.S. Pat. No. 4,605,696 discloses similar highly branched novolak resins composed of resorcinol monoesters, particularly resorcinol monobenzoate. In U.S. Pat. No. 4,892,908 there is disclosed the use of keto derivatives of resorcinol such as benzoylresorcinol. U.S. Pat. No. 5,021,522 discloses aralkyl substituted phenolic resins, particularly aralkyl substituted resorcinol novolak resins with one or more compounds selected from the group consisting of styrene, alpha-methylstyrene, p-methylstyrene, alpha-chlorostyrene, divinylbenzene and vinylnaphthalenes.
The continuing need for a better tackifier with increased tack has long been recognized in the rubber industry. A particular need exists in the tire industry because of the poor tack by synthetic rubber compositions, such as commercial SBR-based tire compositions, particularly after aging for a substantial period of time. Attempts have been made to improve tack and processing of making tackifiers for a quite some time.
This invention answers that need. This invention relates to a modified hydrocarbylphenol-formaldehyde linear novolak resin having increased tack. The invention also provides a process for preparing the modified resin, and an improved rubber composition containing the modified resin.